Intra-familial dispersion patterns in white-footed mice,Peromyscus leucopus

Summary Spatial relationships of white-footed mouse (Peromyscus leucopus) dams and their offspring and of littermates post-weaning were determined to evaluate if their dispersion was sufficient to prevent inbreeding. Mothers tended to leave their first nest site and establish another nest site within or near their original home range. Daughters inherited part of the maternal home range but sons dispersed. During the following breeding season, sons were not in proximity to their mothers or sisters. Daughters, however, frequently attained sexual maturity and bred within the home range of males that could have been their fathers. Differential dispersal of male and female juveniles reduces the probability of mother-son and brother-sister matings, but there is no obvious dispersal-related mechanism to prevent father-daughter matings.     

Parasites prevent summer breeding in white-footed mice, Peromyscus leucopus

Food and parasites can independently play a role in destabilizing population fluctuations of animals, and yet, more than 50 years ago, David Lack proposed that these two factors should act in concert. We examined the role of these factors on the vital rates of free-living white-footed mice (Peromyscus leucopus) over the summer and autumn months. We used a replicated factorial experiment in which deer exclosures doubled acorn availability and anthelmintic application reduced gastrointestinal helminths. Specifically, we wanted to know if either factor or an interaction between the two accounted for the midsummer breeding hiatus observed in this species. We found no influence of habitat quality on mouse breeding, vital rates, or demography; however, anthelmintic treatment resulted in mice continuing to reproduce during the hiatus at the same rate as previously, and they also exhibited increased body condition, growth rate, and survival. These results provide evidence that gastrointestinal helminths reduce P. leucopus reproductive output in central Pennsylvania, and these effects on reproduction could play a role in the unstable dynamics of small mammals.     

Genetic evidence of promiscuity in Peromyscus leucopus

Summary We collected pregnant female Peromyscus leucopus from natural populations during the summer of 1987 and 1988 and allowed these females to give birth to their field-conceived young in the laboratory. Blood samples were taken by suborbital puncture and phenotypes of five genetic loci (Esterase-1, trasferrin, hemoglobin, albumin and 6-phosphogluconate dehydrogenase) were studied using horizontal starch-gel electrophoresis to detect multiple paternity in single litters. Only esterase-1 was found to be highly polymorphic, with four alleles in samples of both years. One litter out of 29 in 1987 and 6 litters out of 32 in 1988 contained three different paternal alleles and served as direct evidence of multiple paternity in the field. The proportion of females engaging in multiple matings in natural populations of P. leucopus, assuming that all males were involved in every multiple mating, is 25%–100% (mean 68%). Because it is unlikely that all males are involved in every multiple mating, the actual proportion of females engaging in multiple matings should be greater. Offprint requests to: X. Xia at his Canadian address     

Mechanisms of sexual fidelity in the monogamous California mouse,Peromyscus californicus

Summary Peromyscus californicus are exclusively monogamous in the wild. We examined in the laboratory whether established pairs of male and female P. californicus would remain faithful when given the opportunity to mate with an opposite sex stranger, either in the presence or absence of their partner. When the partner was present she or he was tethered and could not intervene. Females in postpartum estrus (day 0 postpartum) preferred to associate with their pairmate rather than the unfamiliar virgin male. Females also preferred to mate with their partner, but 15–20% of the females mated with the unfamiliar male both in the presence and absence of their partner. Paired males were tested with an unfamiliar, sexually inexperienced estrous female on days 4 and 8 postpartum with their partners either absent or present but not in postpartum estrus, and on day 0 in the absence of their partner. Males preferred to associate with their pairmate. Contrary to theoretical expectations, males did not copulate with the estrous female when given the opportunity regardless of whether their partner was present or not. Furthermore, few males (2/13) tested in the absence of their partner mated with a sexually experienced, postpartum estrous female. These results indicate that monogamy in P. californicus is maintained by a strong attraction and preference of pairmates for each other and by self-restraint from mating with others. Male mate guarding may further ensure female faithfulness. Males exhibit apparently more sexual fidelity than females.Correspondence to: D.J. Gubernick     

Selection for male choice based primarily on mate compatibility in the oldfield mouse, Peromyscus polionotus rhoadsi

Despite the consensus that mate choice acts as a mechanism for selection of secondary sexual traits, the evolutionary forces affecting mate preferences themselves remain controversial. In this study, we first demonstrated selection acting directly on the mate preferences of monogamous male oldfield mice, Peromyscus polionotus rhoadsi. One group of male oldfield mice were allowed to express a social preference between two potential mates, and were subsequently paired with either their preferred or rejected female. Among these pairs, those containing preferred females produced more offspring than did those containing rejected females. We next demonstrated that this fitness advantage depended primarily on compatibility between the members of a mated pair. A second group of male oldfield mice were not allowed the opportunity to express a social preference between potential mates. Rather, these males were paired with females that had been either preferred or rejected by males in the first group. Among these pairs, those containing preferred females did not produce more offspring than those containing rejected females. In other words, individual mate preferences had fitness consequences only for those males that expressed them, demonstrating that these preferences were based primarily on compatibility between mates.     

Individual differences in the mechanisms underlying the onset and maintenance of paternal behavior and the inhibition of infanticide in the monogamous biparental California mouse,Peromyscus californicus

Virgin male Peromyscus californicus tend to behave infanticidally or nonparentally towards pups, whereas virtually all males exhibit parental behavior following birth of their own young. Most males (65–75%) living with their pregnant partner attacked or ignored unfamiliar pups and did not become parental (behave parentally) prior to birth of their young. However, a significant minority of males (34%) living with their partner became parental after just 24 h of postcopulatory cohabitation with the female and remained parental throughout their mate's pregnancy. Males that were infanticidal before the birth of their young became parental only after their young were born. The presence of the mother was necessary for the postpartum maintenance of paternal behavior and the inhibition of infanticide in males that were infanticidal prepartum. In contrast, males that were paternal prior to birth of their young continued to be paternal after birth, even in the absence of postpartum contact with the mother. Thus, different mechanisms are involved in the inhibition of infanticide and the onset and maintenance of paternal behavior. Correspondence to: D.J. Gubernick     

Social organization and nest co-occupancy in Peromyscus californicus,a monogamous rodent

Summary Home ranges, social organization, and nest co-occupancy of Peromyscus californicus were studied using radiotelemetry at the Hastings Natural History Reservation, California. Mated pairs were ascertained by the transfer of fluorescent pigments from lactating females to putative fathers. Mated pairs had largely overlapping home ranges that were not statistically distinguishable, whereas adjacent adults had mostly exclusive, statistically distinguishable home ranges. There was no difference in the mean home range of males and females, but mated females tended to have smaller ranges than their mate. Home range size was extremely variable (range: 150–3788 m²) and averaged 1161 m² across all individuals. Male home range size was inversely correlated with population density, suggestive of a social influence on home range. Putative fathers spent comparable amounts of time to females in the nest — presumably caring for the young — which supports previous laboratory reports of paternal care in this species. All data collected in this study are consistent with previous suggestions that P. californicus live in semi-permanent family groups and are monogamous. Offprint requests to: D.O. Ribble     

The effects of mating on infanticide and pup discrimination in white-footed mice

Summary A laboratory study was conducted with white-footed mice (Peromyscus leucopus) to determine (1) if cohabitation and/or successful mating inhibits infanticide, and if so for how long, and (2) if males and females can discriminate between their own (familiar) and strange (unfamiliar) pups. Infanticidal males and females that were paired for 15 days and did not reproduce were not inhibited from killing pups, whereas infanticidal males and females that were paired until they reproduced were inhibited from committing infanticide at 4 days postpartum. Neither males nor females discriminated between familiar and unfamiliar pups with regard to engaging in infanticide. Approximately 50% of wild-caught males and females killed pups in the laboratory. Infanticidal pairs bred significantly sooner than did noninfanticidal pairs. The results are consistent with the promiscuous nature of P. leucopus males and territoriality of P. leucopus females. Offprint requests to: Donna M. Cicirello, Kellogg Biological Station, 3700 E. Gull Lake Drive, Hickory Corners, MI 49060, USA     

Immune defense and reproductive pace of life in Peromyscus mice

Immune activity is variable within and among vertebrates despite the potentially large fitness costs of pathogens to their hosts. From the perspective of life history theory, immunological variability may be the consequence of counterbalancing investments in immune defense against other expensive physiological processes, namely, reproduction. In the present study, we tested the hypothesis that immune defense among captive-bred, disease-free Peromyscus mice would be influenced by their reproductive life history strategies. Specifically, we expected that small species that reproduce prolifically and mature rapidly (i.e., fast pace of life) would favor inexpensive, nonspecific immune defenses to promote reproductive proclivity. Alternatively, we expected that large species that mature slowly and invest modestly in reproduction over multiple events (i.e., slow pace of life) would favor developmentally expensive, specific immune defenses and avoid cheap, nonspecific ones because such defenses are predisposed to self-damage. We found that species exhibited either strong ability to kill (gram-negative) bacteria, a developmentally inexpensive defense, or strong ability to produce antibodies against a novel protein, a developmentally expensive defense, but not both. Cell-mediated inflammation also varied significantly among species, but in a unique fashion relative to bacteria killing or antibody production; wound healing was comparatively similar among species. These results indicate that Peromyscus species use immune strategies that are constrained to a dominant axis, but this axis is not determined solely by reproductive pace of life. Further comparisons, ideally with broader phylogenetic coverage, could identify what ecological and evolutionary forces produce the pattern we detected. Importantly, our study indicates that species may not be differentially immunocompetent; rather, they use unique defense strategies to prevent infection.     

Benthic-planktonic coupling, regime shifts, and whole-lake primary production in shallow lakes

Alternative stable states in shallow lakes are typically characterized by submerged macrophyte (clear-water state) or phytoplankton (turbid state) dominance. However, a clear-water state may occur in eutrophic lakes even when macrophytes are absent. To test whether sediment algae could cause a regime shift in the absence of macrophytes, we developed a model of benthic (periphyton) and planktonic (phytoplankton) primary production using parameters derived from a shallow macrophyte-free lake that shifted from a turbid to a clear-water state following fish removal (biomanipulation). The model includes a negative feedback effect of periphyton on phosphorus (P) release from sediments. This in turn induces a positive feedback between phytoplankton production and P release. Scenarios incorporating a gradient of external P loading rates revealed that (1) periphyton and phytoplankton both contributed substantially to whole-lake production over a broad range of external P loading in a clear-water state; (2) during the clear-water state, the loss of benthic production was gradually replaced by phytoplankton production, leaving whole-lake production largely unchanged; (3) the responses of lakes to biomanipulation and increased external P loading were both dependent on lake morphometry; and (4) the capacity of periphyton to buffer the effects of increased external P loading and maintain a clear-water state was highly sensitive to relationships between light availability at the sediment surface and the of P release. Our model suggests a mechanism for the persistence of alternative states in shallow macrophyte-free lakes and demonstrates that regime shifts may trigger profound changes in ecosystem structure and function.     

Pregnancy blockage following multiple-male copulation or exposure at the time of mating in deer mice,Peromyscus maniculatus

Summary Pregnancy blockage resulting from multiple-male copulation or exposure around the time of mating was studied in three experiments on deer mice, Peromyscus maniculatus. First, significantly more females were pregnant after copulating, either with or without disturbance and a delay, with one male than after receiving the same total number of ejaculations from two males. Second, when females received three ejaculations from one male and then were immediately exposed to either a blonde male or a wild-type male for 2 h, the pregnancy rate was lower than when they were exposed to an empty cage. Third, when immediately after mating females were placed for 2 h in the cage of a strange male they were less likely to deliver a litter than when placed in the familiar male's cage. This phenomenon bears at least a superficial resemblence to the Bruce effect but either communality or difference in underlying mechanism remains to be determined. The present multi-male pregnancy block may provide a means for females in unstable social situations to defer reproduction for 4 days or longer when more stable conditions may exist and a single male prevails.     

Male capacity as related to sperm production,pregnancy initiation,and sperm competition in deer mice (Peromyscus maniculatus)

Summary Limitations on male capacity may be important to the evolution of mating strategies and behavior. Sperm counts in successive ejaculates of deer mice (Peromyscus maniculatus) decreased progressively through six ejaculations and remained depressed the following day. Males were as effective in inseminating females in cycling estrus with their second two ejaculates as with their first two and as effective in inseminating postpartum females with their fifth as with their first. In sperm competition situations males competed with rested males as effectively with their fourth, fifth, and sixth ejaculates as with their first. Thus, although sperm counts decrease in successive ejaculates, males suffer no apparent functional deficiency as long as they continue ejaculating. It is not clear whether or not deer mice are honest salesmen but they appear to be honest ejaculators and usually are honest intromitters.     

Precise sex allocation, local mate competition, and sex ratio shifts in the parasitoid wasp Trichogramma pretiosum

We determined the sex, order, and clutch size of eggs laid by the parasitoid wasp, Trichogramma pretiosum Riley, in the eggs of one of its natural hosts, Trichoplusia ni (Hübner). The parasitoid allocated sex non-randomly to hosts in the laboratory with a variance significantly less than that of a binomial (random) distribution, our null model. More clutches of two or more eggs contained a single male egg as the second or third egg laid than would be expected by chance and none contained two or more male eggs. T. pretiosum also increased the sex ratio (% male) of its offspring with increasing foundress numbers by increasing the frequency of male offspring as the second egg in a two-egg clutch allocated to unparasitized hosts and as the single egg allocated to previously parasitized hosts. These results indicate that T. pretiosum allocates the sex of its offspring precisely. Precise sex allocation is favored under local mate competition because it reduces variation in the number of sons per patch thus maximizing the number of inseminated daughters emigrating from the patch. Similar combinations of female and male offspring emerged from T. ni eggs parasitized by T. pretiosum in the field, again with a sex ratio variance less than that expected for a binomial distribution. These results strongly suggest that this parasitoid species manifests local mate competition.     

Influence of corallivory, competition, and habitat structure on coral community shifts

The species composition of coral communities has shifted in many areas worldwide through the relative loss of important ecosystem engineers such as highly branched corals, which are integral in maintaining reef biodiversity. We assessed the degree to which the performance of recently recruited branching corals was influenced by corallivory, competition, sedimentation, and the interactions between these factors. We also explored whether the species-specific influence of these biotic and abiotic constraints helps to explain recent shifts in the coral community in lagoons of Moorea, French Polynesia. Population surveys revealed evidence of a community shift away from a historically acroporid-dominated community to a pocilloporid- and poritid-dominated community, but also showed that the distribution and abundance of coral taxa varied predictably with location in the lagoon. At the microhabitat scale, branching corals grew mainly on dead or partially dead massive Porites ("bommies"), promontories with enhanced current velocities and reduced sedimentation. A demographic study revealed that growth and survival of juvenile Pocillopora verrucosa and Acropora retusa, the two most common branching species of each taxon, were affected by predation and competition with vermetid gastropods. By 24 months of age, 20-60% of juvenile corals suffered partial predation by corallivorous fishes, and injured corals experienced reduced growth and survival. A field experiment confirmed that partial predation by corallivorous fishes is an important, but habitat-modulated, constraint for branching corals. Competition with vermetid gastropods reduced growth of both branching species but unexpectedly also provided an associational defense against corallivory. Overall, the impact of abiotic constraints was habitat-specific and similar for Acropora and Pocillopora, but biotic interactions, especially corallivory, had a greater negative effect on Acropora than Pocillopora, which may explain the local shift in coral community composition.     

Maintenance of familiarity and social bonding via communal latrine use in a solitary primate (Lepilemur leucopus)

Latrine use (i.e., the repeated use of specific defecation/urination sites) has been described for several mammals, including carnivores, ungulates, and primates. However, the functional significance of latrine use in primates has not been studied systematically yet. We, therefore, followed 14 radio-collared individuals of the pair-living white-footed sportive lemur (Lepilemur leucopus) for 1097 hours of continuous focal observations to investigate latrine distribution, seasonality of latrine use, as well as age and sex of users to test various hypotheses related to possible functions of latrine use, including territory demarcation, resource defense, signaling of reproductive state, social bonding, and mate defense. All individuals of a social unit exhibited communal use of latrines located in the core area of their territory, supporting the social boding hypothesis. Latrine use seems to facilitate familiarity and social bonding within social units via olfactory communication in this primate that lives in family units but exhibits low levels of spatial cohesion and direct social interactions. In addition, frequency of latrine visitation was higher during nights of perceived intruder pressure, supporting the mate defense hypothesis. However, animals did not react to experimentally introduced feces from neighboring or strange social units, indicating that urine may be the more important component of latrines than feces in this arboreal species. Based on a survey of latrine use and function in other mammals, we conclude that latrines facilitate communication particularly in nocturnal species with limited habitat visibility and in species where individuals are not permanently cohesive because they constitute predictable areas for information exchange.     

Ontogenetic feeding shifts in the meiobenthic harpacticoid copepod Nitocra lacustris

¹⁴C-radiolabelling experiments indicate that adult stages of the salt-marsh harpacticoid copepod Nitocra lacustris (Schmankevitsch) receive a large part of their nutrition through the ingestion and assimilation of certain diatoms. An abundance of empty diatom frustules occurs in the gutpellet contents of field-collected individuals. Naupliar stages do not ingest diatoms in the laboratory, and nauplii from the field do not contain frustules in their gut pellets. Ingestion of diatoms in the laboratory first occurs during the second or third copepodite stage. ³H-radiolabel expeiments and grazing experiments using bacterium-sized beads adhering to the diatoms indicated that both adults and nauplii ingest bacteria adhering to the outer mucus coating of the diatoms (and probably ingest the diatom mucus itself). Adults ingest bacteria (and probably mucus exopolymer) coincidently while ingesting diatoms. The nauplii ingest these components by scraping the outer surface of the diatoms. SEM observations indicate that diatoms are not punctured by the nauplii during feeding. While diatom mucus and associated bacteria play an (as yet unquantified) role in the nutrition of the adults, these components may comprise the bulk of food resources for naupliar stages.     

Spatial variation and ontogenic shifts in the isotopic composition of Mediterranean littoral fishes

Stable carbon and nitrogen isotope signatures have been determined for littoral fishes from three localities in the Balearic Islands (NW Mediterranean) for trophic assessment. Variation in the isotopic composition was examined at multiple levels: the individual organism, individual populations (ontogenetic variation) and at area effects (different islands). Cluster analyses based on ¹³C and ¹⁵N resulted in two very different dendrograms for the fish assemblages from Menorca and Formentera. In both schemes, congeners of the genera Diplodus and Labrus did not cluster together, suggesting that closely related species do not necessarily feed on similar foods. Significant differences in ¹⁵N were detected between locations for Scorpaena porcus, Scorpaena scrofa and Sciaena umbra, and significant differences in ¹³C were detected for Scorpaena porcus and Scorpaena scrofa. Overall there were significant spatial differences in the trophic position of littoral fishes among the three Balearic Islands, and fishes from Palma, Mallorca tended to feed at a higher trophic level than fishes from either Menorca and Formentera. Large-scale geographic comparisons, using earlier data from three sites in western Mallorca and in Corsica revealed that throughout the western Mediterranean generally, the average trophic level of fishes differs remarkably little. However, large discrepancies were observed in ¹⁵N for individual species, notably Scorpaena porcus, D. annularis, D. vulgaris and Serranus scriba. ¹³C composition of fishes did not seem to change greatly among the locations, although marked discrepancies were observed for D. puntazzo and Mullus surmuletus. Given the large-scale patterns we have observed, it is possible that the extent of multichannel omnivory which has been purported to exist in complex aquatic systems, may have been greatly overstated. Regression of ¹⁵N and ¹³C with fish body size revealed clear and significant trends for some species. In particular, ¹⁵N appeared to increase with body length in the macrocarnivorous fishes Scorpaena porcus, Scorpaena scrofa, Serranus scriba and Sciaena umbra. Trends were less apparent in animals which feed primarily on small benthic invertebrates, notably the four Diplodus species (D. vulgaris, D. sargus, D. annularis, D. puntazzo), and the labrids Labrus viridis and L. merula.Communicated by J.P. Thorpe, Port Erin     

Isotopic shifts with size, culture habitat, and enrichment between the diet and tissues of the Japanese scallop Mizuhopecten yessoensis (Jay, 1857)

Use of stable isotope signatures to trace diet patterns in cultured marine bivalves, particularly when changing culture habitat, requires knowledge of the isotopic shift and enrichment between diet and consumer’s tissues. The aim of this study was to determine the patterns of isotope change and the variability of enrichment values (∆δ¹³C and ∆δ¹⁵N) in different tissues (muscle, gonad, digestive gland) of the Japanese scallop (Mizuhopecten yessoensis). It was hypothesized that the isotopic signatures of a consumer’s tissues changed during settlement and that the changes were related to variations in the isotopic signatures of food sources and gut contents. Particular attention was paid to the isotope enrichment between the diet and a consumer’s tissues using isotope analysis of gut content. Muscle δ¹⁵N values decreased significantly 3–5 months post-settlement in a nearshore seabed, concomitant with the ingestion of lower δ¹⁵N food. For juvenile scallops, sinking particles (SP) were considered a more important dietary source than suspended particulate organic matter (SPOM), based on the correspondence between SP and gut contents δ¹³C. Enrichment values (∆δ¹³C and ∆δ¹⁵N) varied with tissue and season. ∆δ¹⁵N was 2.4‰ in muscle, 1.2‰ in gonad, and 0.7‰ in the digestive gland. ∆δ¹³C was 3.2‰ in muscle, 2.3‰ in gonad, and −0.5‰ in the digestive gland. ∆δ¹⁵N was the lowest in summer (0.3‰), and ∆δ¹³C was the highest in autumn (2.8‰). ∆δ¹⁵N values were significantly influenced by age, but not ∆δ¹³C. Patterns of isotope ratios and enrichment values may be related to physiological attributes and differences in diet. This is the first study to demonstrate isotopic shift and enrichment encountered in different tissues of a cultured scallop when changing culture habitat.     

Distribution of thifluzamide,fenoxanil and tebuconazole in rice paddy and dietary risk assessment

Distribution behaviors of thifluzamide, fenoxanil, and tebuconazole applied to rice were investigated in South China. Analysis was by a modified QuEChERS method with gas chromatography (thifluzamide and fenoxanil) and liquid chromatography mass spectrometry (tebuconazole). Thifluzamide and tebuconazole partitioned mainly into the soil, with half-lives in the paddy soil of 12–14 and 5.3–7.8 days, respectively. Fenoxanil partitioned mainly into the rice plants, with half-lives of 3.3–4.4 days. The half-lives of thifluzamide, fenoxanil, and tebuconazole in paddy water were 0.17–0.89, 1.8–3.0, and 1.6–4.0 days, respectively. The residues in rice grains at the pre-harvest interval of 14 days were all below the established maximum limit values. The dietary risks assessed as hazard quotients at the pre-harvest interval were less than 1.